Answer:
Valves prevent the backward flow of blood.
Explanation:
The heart consists of two atria (upper chambers) and two ventricles (lower chambers). There is a valve through which blood passes before leaving each chamber of the heart. In case of veins, valves keep blood moving in one direction: back toward the heart. For example when the leg muscles are activated, the venous valves open to allow one-way flow in the direction of the heart.
